Mechanisms and pathways involved in the inhibition of pituitary LH release will be investigated in ovariectomized rats. Activation of LH-inhibiting systems which contain opiate receptors will be accomplished either by electrical stimulation or by systemic or intracerebral administration of morphine, and the resulting changes in LRF release will be assessed either by radioimmunoassay of circulating LH levels or by the recording of hypothalamic electrical activity. Spontaneous activity of these inhibitory pathways will be compared in ovariectomized rats in the following situations: a) high tonic LH levels resulting from the release of LH secretion from the inhibitory feedback of gonadal steroids; b) low tonic LH levels following treatment with estrogen; and c) high phasic LH levels resulting from the simulation of a preovulatory surge by stimulatory feedback of estrogen.